The present invention relates to a method of controlling a ratio change in a continuously variable transmission.
A conventional method of a ratio change in continuously variable transmission, is disclosed in Laid-open Japanese Patent Application No. 57-90450. According to this conventional method, a control unit stores an ideal control line which interconnects points disposed on the performance curves of an engine, each point representing an operating condition under which the engine can operate with the least fuel consumption rate. Comparison is made between the operating condition represented by a corresponding one of the points on the ideal control line and an actual operating condition (an engine revolution speed, a throttle opening degree, a vehicle speed and etc.) of a vehicle. In response to the result of this comparison, the ratio change is controlled in such a manner as to bring the actual operating condition of the engine into agreement with the stored operating condition on the ideal control line. The ratio change is carried out within a ratio change allowable range which is fixed and determined according to the design of the pulley drive unit of the transmission. Taking into account the manufacturing variation of the pulley drive unit and the change according to aging, the ratio change allowable range which is actually used in carrying out the ratio change is set narrower than a ratio change allowable range, which is actually usable. Viz., in order to prohibit the issuance of the instructions to establish a ratio outside of the range actually attainable by the pulleys, the relatively narrow ratio change allowable range is necessarily set. Thus, the conventional continuously variable transmission has to operate on the relatively narrow ratio change allowable range. As a result, a problem arises that a sufficiently large acceleration is not produced when the vehicle moves off from standstill because a reduction ratio large enough for the sufficient acceleration is not allowed to use, and a sufficient improvement in the fuel consumption rate is not made during stable operating state at high speeds because a sufficiently small reduction ratio is not allowed.